Quantum Well Design for Red-Emitting InGaN LEDs
The quantum well design is incorporated within a red-emitting InGaN QW LED, an LED type that usually cannot efficiently emit wavelengths larger than blue/green. The LED can be incorporated into a same-material RGB platform for white LEDs, because the well design increases efficiency and decreases indium composition compared to similar wavelength InGaN LEDs.
Blue-emitting InGaN quantum well (QW) LEDs now have 69% external quantum efficiency, but increasing the emitted-wavelength decreases the efficiency significantly. The inversion asymmetry of the InGaN lattice increases the induced strain, which in turn increases the piezoelectric effect. Charge builds in the QW, inducing an electrical field, which causes the QW to no longer produce discrete sublevels. Multiple wavelengths of photons are emitted, instead of just one. The emitted light is less intense at one wavelength and more electrical current must be passed through the QW to recombine holes and electrons at the same rate.
Red-emitting InGaN QW LEDs are not only inefficient, but to reach a red wavelength, the composition of indium must be increased. Indium often comes in a low quality material with a high number of defects. Research to increase efficiency has focused on using less polar InGaN, staggering InGaN QW, strain-compensating with InGaN/AlGaN QW, using type-II InGaN/GaNAs QW, and using InGaN-Δ-InN QW. A same-material RGB platform for white LEDs is commercially impractical for conventional InGaN QW LEDs.
LED platforms are used in electronic devices and screens such as mobile phones, tablets, TVs, computers, computer monitors, watches and even streetlamps and vehicle headlights. Inefficient emission at longer wavelengths limits the color combinations of the electronic screens, depletes battery life, or decreases the lifetime of the LED due to charge build up. For white LEDs without an RGB platform, light is produced using phosphor. Small changes in the phosphor layer drastically changes the color temperature of the light. Most white LEDs using phosphor have a cool color temperature which constricts the pupil and hurts eyesight.
Researchers at the Ohio State University, led by Dr. Hongping Zhao have developed novel QWs for light emission beyond the blue and green spectra. The QW has layers of InGaN, ZnSnN2, and AlGaN to increase lattice-matching and hole-electron recombination of the red-emitting LED. The layering has better hole and electron confinement but with increased electron-hole wave function overlap. As a result, the holes and electrons can recombine more efficiently and the QW sublevels remain discrete. The InGaN QW LED can efficiently emit red light (685 nm) with a higher intensity and lower indium composition.